Calcium Metasilicate. 105 



tions after the Bertrancl-Mallard method. In certain sections 

 the optic axial angle appeared to be smaller than in others, a 

 fact for which no explanation has been found. Optic axial 

 dispersion p > v. All of the above properties agree well with 

 those of natural wollastonite and substantiate the thermal and 

 chemical evidence of Messrs. Allen and White. 



Several of the preparations of wollastonite were crypto- 

 crystalline and could be identified only by their low birefring- 

 ence and fibrous spherulitic character. 



Pseudo-wollastonite appears either in the form of small 

 irregular grains often tabular in shape or in short prisms or 

 fibers arranged in parallel or divergent groups. The grains 

 are transparent and show occasionally well developed basal and 

 imperfect prismatic cleavage lines. Limiting refractive indices, 

 about 1*615 and l - 64:5, as measured by the method of refractive 

 liquids. Birefringence about - 025-0 - 035, considerably higher 

 than in wollastonite. It is interesting to note in this connec- 

 tion how slightly the mean refractive index of wollastonite 

 differs from that of pseudo-wollastonite. Accurate optical 

 measurement would be required to ascertain satisfactorily 

 which mineral has the higher average index of refraction. 

 The same conditions prevail in their specific gravities, where 

 the differences observed might well be ascribed to experi- 

 mental error. 



Optical character, positive, with very small optic axial angle 

 2 E = 0° — 8°. Plates cut parallel to the basal pinacoid show 

 in convergent polarized light an interference cross which can 

 often be seen to open slightly on turning the stage. The optic 

 axial angle thereby is so small that from it alone the mineral 

 might be regarded uniaxial, the mere opening of the interfer- 

 ence cross being an optical anomaly analogous to the irregu- 

 larities noted in many minerals. This is the view taken 

 by J. H. L. Vogt* in his studies on the formation of minerals in 

 slags. His opinion was strengthened by the hexagonal form 

 of the crystals and by the observed extinction parallel to the 

 basal pinacoidal cleavage cracks. Doelterf, who also made an 

 extended microscopic study of this silicate, came to the conclu- 

 sion that the mineral was either hexagonal or orthorhombic in 

 crystal system, his observations agreeing otherwise with those 

 of Yogt. Bourgeois,;}; on the other hand, pronounced the min- 

 eral monoclinic. His work was accomplished before that of 

 Doelter and Vogt and appears to have been less extensive in 

 scope. After a brief mention of the essential optical features 

 he describes the occurrence of twinning lamellae in certain of the 



* Mineralbildung in Schmelzmassen, Kristiania, 1892, 57-59. 

 f N. Jahrb. f. Min. 1886, i, 119-122. 

 J Bull. Soc. Min., v, 14-15. 



Am. Jour. Sci.— Fourth Series, Vol. XXI, No. 122. — February, 1906. 



